Four Weddings and a Funeral (1994 film) = Finished

Four Weddings and a Funeral has one of the saddest happy endings I've ever seen. Charles and Carrie, standing in the rain, pledging their love amidst a dedication to never marry each other.

What's sad about this ending isn't that they didn't, well, marry - it's that we weren't given a reason to  care about one of the two most important people in this fictional world.

Carrie isn't given a...Carriecter.

The traditional structure of a romantic comedy shows us two people, and then illustrates all of the many tiny reasons they should be together until they add up to inevitability. When obstacles are thrown in their way, we care as an audience. We feel righteous outrage when their own communication issues block eternal love. All because these two people - the most important people in this world - are PERFECT for each other. It's meant to be. It has to happen.

We know, because we know them.
This isn't the case in Four Weddings and a Funeral.

Carrie is a cipher. A woman-shaped object. She talks about how many people she's had sex with, and she clearly has sexy fun times with Charles. She is the object of his affections, desire, and possibility of not being alone for the rest of his natural life. However, we never really learn about Carrie as a person, in hard contrast to pretty much every other character in the movie. Given this context, the absence is glaring. Almost everyone has an inner truth or core of empathy that is revealed during the course of the story, except for the woman we need to root for the most. 

Any one of the women surrounding Hugh Grant's character - Scarlett, Veronica, maybe even "Pigface" Henrietta if they really wanted to bend the story that direction - could have been given a few extra scenes to further their human qualities, and theoretically turn them into the love interest. That didn't happen. Carrie doesn't get this development either, so we're instead left with this void that only furthers as Carrie ultimately marries another man.

A pledging of love and loyalty in the rain, supposedly destined to be, somehow becomes one of the saddest happy endings ever.